Obesity among Britain's pets has become so acute that three-quarters of vets now run fat clinics to combat the problem, a survey revealed today.

At a time of growing concern about obesity among children, the poll by the Royal Society for the Prevention of Cruelty to Animals showed that the junk-food-induced crisis had spread to pets and was getting worse.

Four out of five vets questioned said the number of overweight and obese animals they were seeing was on the increase. Of the 143 veterinary practices surveyed, 76% said they held regular sessions for fat pets.

The news comes after Britain's first dedicated weight loss clinic for pets opened at the University of Liverpool small animal hospital earlier this year.

The RSPCA is urging owners to watch the weight of their pets and to stop feeding them junk food. It cited the case of a dog that had grown to almost double its normal weight after being fed a diet that included breakfast fry-ups.

RSPCA inspectors reported several cases last year in which animals had suffered from being overweight or obese. Most of the cases involved dogs, but they included a donkey and even a parrot.

The poll of vets showed that 84% of owners were surprised when told their pets had a weight problem.

An accompanying survey of 2,300 owners found that almost one-third thought their pets were overweight. More than one-quarter of these said they had done nothing to address the problem.

An RSPCA checklist for owners says that, as a general rule, if you cannot feel your pet's ribs it is overweight. It recommends increased exercise for pets and a calorie-controlled diet.

An RSPCA vet, David Grant, said: "Feeding your pet full English breakfasts, chocolate, chips, burgers and too much pet food may make you feel like a kind and loving owner, but it may [also] give your pet fatty tumours, diabetes, arthritis, high blood pressure, liver failure, skin problems and, even worse, an early death."

He added: "Pet obesity is a serious welfare problem and, as a nation of so-called animal lovers, we have a duty to tackle it now."